The proposed research aims at a better understanding of the molecular mechanism of steroid hormone action. Steroid hormones interact with a class of regulatory proteins, steroid hormone receptors, which play an important role in the development and in the physiological regulation of all higher animals. Steroid hormone receptors appear to be the most experimentally accessible candidates for studying gene regulation in eukaryotic cells. The glucocorticoid receptor is chosen here for investigation, since a genetic approach has been possible. The glucocorticoid receptor, like other steroid receptors, constitutes only a minute proportion of the total cellular proteins, which makes it extremely difficult and expensive to purify it in amounts sufficient for a full biochemical analysis. It would, therefore, be extremely advantageous if the glucocorticoid receptor gene could be cloned. Recent progress in DNA-mediated gene transfer now provides techniques for gene isolation for which appropriate selective conditions and recipient cells exist. Our previous genetic approach toward the analysis of the glucocorticoid receptor in mouse lymphoid cell lines has provided us with a unique set of cell lines and a selection procedure which can now be used to isolate the mouse glucocorticoid receptor gene. In the next few years, I plan to isolate and characterize the mouse glucocorticoid receptor gene. This specific probe will be used to purify and clone cDNA from glucocorticoid receptor mRNA and its analysis will allow conclusions about the protein it codes for. Most recent techniques developed for DNA-mediated gene transfer and cloning techniques will be used. The long-range objective is to obtain a more detailed understanding of steroid hormone-receptor interaction and the mechanism by which steroid receptors exert their regulatory function. The system chosen here offers unique tools to reach these goals since it is the only system in which a genetic approach is possible; in the best studied systems, the progesterone and estrogen receptors, mutations affecting the receptors cannot be selected.